Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Line tangent to the curve

  1. Oct 26, 2008 #1
    1. The problem statement, all variables and given/known data

    For what values of a and b is the line 3x+y=b tangent to the curve y=ax^3 when x=5 ?

    3. The attempt at a solution
    I believe i would need to find the equation of the tangent to y=ax^3 when x=5 then that should be equal to -3x+b=y if im not mistaken but im not sure what to do from here to solve for a and b.

    Any leads in the right direction or insight if this is the entirely wrong approach would be greatly appreciated!
  2. jcsd
  3. Oct 26, 2008 #2


    User Avatar
    Science Advisor

    Well, yes, that's just a restatement of the problem! You know that the slope of y= -3x+ b is -3. What connection is there between the slope of a tangent line and the function itself?
  4. Oct 26, 2008 #3
    thank you for the quick reply hallsofivy. I had -0.04 instead of -3/75 giving me the wrong answer for the online homework webpage which was confusing me. After using -3/75 instead it counts it as correct :(
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook